Cathode-ray tubes



A. v. DE VERE KRAUSE CATHODE-RAY TUBES Filed June 22, 1956 A ril 1 1958 3 mm b oczooo 9:29.60

ANTHONY VlCTOR-DEVERE KRAUSE INVENTOR.

HIS ATTORNEY.

United States Patent 2,829,292 CATHODE-RAY TUBES Anthony Victor de Vere Kranse, Ashtead, England, assignor to Cinema-Television Limited, London, England, a British company Application June 22, 1956, Serial No. 593,137 Claims priority, application Great Britain July 4, 1955 3 Claims. (Cl. 31332) on adjacent electrodes and avoltage surge occurs, and

such discharge can present a formidable problem particularly when operating with anode-cathode potentials in the order of jkilovolts.

Dielectric breakdown within the cathode-ray tube presents two undesirable effects. First, should the high discharge current circulate through the grid circuit, voltages suificient to damage circuit components are developed with accompanying deleterious results in the cathode circuit because of the increased cathode current due to the high grid potential. Secondly, the spark discharge may cause material to be sputtered from thesurfaces between which the discharge takes place. In addition'to damage to the affected surfaces, some of the sputtered (conductin material may be deposited on adjacent insulating surfaces. I

' In order to illustrate the magnitude of current that can be produced by such a discharge, the following analysis may be helpful. Assume the duration of spark discharge to be 1 microsecond, and the capacity of the screen and the envelope to be in the order of .001 microfarad. This capacity when subject to a 25-kilovolt anode potential will support a charge of 25 microcoulombs which, if discharged in 1 microsecond, will supply an average current of 25 amperes, with peakcurrents that may well be 10 to 100 times greater.

A practical solution currently utilized to combat the aforementioned problems requires protective impedances in those external circuits which may be subjected to high discharge currents. This solution, however, does not endear itself to those applications where economy is an important factor.

Accordingly, it is an object of the present invention to provide an improved cathode-ray tube which avoids the above-mentioned disadvantages of the prior art.

It is a further object of this invention to provide a cathode-ray tube having built-in provision for obviating those deleterious effects resulting from high discharge currents.

It is also an object of this invention to accomplish such protective means in an economical and expeditious manner.

A cathode-ray tube constructed in accordance with this invention includes, within an evacuated envelope having a neck portion and a flared portion, a luminescent screen and means including a multielectrode electron gun disposed within the neck portion for projecting an electron beam at the luminescent screen. A low-resistance final anode ice encompasses the path of the electron beam throughout substantially theentire length of the aforementioned flared portion of the envelope and is located between the electron gun and the luminescent screen; High-resistance means interconnecting the final anode and one of the electrodes of the electron gun is provided for maintaining the anode and the aforementioned electrode at substantially the same potential while limiting spark discharge currents in the electron gun.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with the accompanying drawing, in which the single figure is a side view, partly schematic and partly in section, of an illustrative embodiment of the invention. In the drawing, certain elements of the tube are shown considerably out of proportion with respect to other elements for the purpose of clearness of illustration.

As shown in the accompanying drawing, a cathode-ray tube constructed in accordance with this invention comprises an evacuated envelope 10 composed of an elongated neck sectionll and a flared cone portion 12; A'luminescent screen 13 is deposited upon an inner surface of the end wall or face plate 14. An electron gun 15 is situated within; elongated neck portion 11 and may be of any conventional construction comprising at least a cathode 16, a control electrode 17 and an accelerating anode 18. The tube further comprises a circumferentially disposed conductive layer 21, which may be formed of vColloidal graphite and is conventionally designated the final anode, within flared portion 12. Final anode 21, which substantially encompasses the path of the electron beam, can be energized from a high potential source (not shown) througha terminal 22 in conjunction with contact 23. Circumferentially disposed adjacent to final anode 21 upon the outside surface of flared portion 12 is a conductive coating 24 whichin conjunction with final anode21 form the plates of a filter capacitor 25 of which cone portion 12 constitutes the dielectric and which is connected to a reference potential 26 such as ground. Cathode 16 and control electrode 17 are also returned to reference potential 26 through resistors 27 and 28, respectively.

In accordance with the invention, a high-resistance coating 20 is provided in contact with final anode coating 21 and extending into the neck portion 11 of the tube. Coating 20 may be formed of black iron oxide or cadmium oxide which can be applied in such thickness that in a tube having, for example, a neck diameter of 35 millimeters, a resistance of approximately 1 megohm is provided by a layer extending over 2 or 3 inches of the neck. In the fabrication of the tube, high-resistance coating 20 may be conveniently formed first and final anode coating 21 may then be applied to overlap coating 20 to furnish the required electrical connection. Accelerating anode 18 is electrically connected to high-resistance coating 20, as by means of conventional contact springs 19.

In operation, a stream of electrons originating at cathode 16 is modulated by control electrode 17 and accelerated by anode 18 to impinge upon luminescent screen 13 to produce a desired image or wave form. Of course, conventional focusing and deflecting elements (not shown) may be provided. I

In conventional tubes, high potential is applied to ac celerating electrode 18 by extending the conductive coating which forms final anode 21 into the neck portion 11 for engagement with resilient contacts 19 directly translate high voltage to anode 18. The purpose of capacitor 25 being to filter the high voltage presented to terminal 2,s29,ass

1 wise apparent that should dielectric breakdown occur between the anode 18 and control electrode 17 or cathode 16 in the absence of resistive coating 20, high peakcurrents will circulate through their associated circuitry, herein indicated as resistors 28 and 27, respectively.

However, high surge currents caused by spark discharge and dielectric breakdown are precluded by the invention through the provision of resistive coating 20 interposed between final anode 21 and resilient contacts 19. This resistive coating 20 forms anextension of final anode 21, and, for purposes of illustration and not to be construed as a limitation, may conveniently have a resistance of the order of l megohm. With a, built-in resistance of 1 megohm included between contacts 19 and final anode 21, the average value of current which, results from a 1 microsecond spark discharge of 25 microcoulombs will be 25 milliamperes, a reduction of 1,0002], as compared to the 25 ampere average in the absence of the resistive coating.

Since the current collected by anode 18, as distinguished from that collected by the luminescent screen 13, is not normally greater than about 100 microamperes, the voltage dropproduced by this anode current flowing through resistive coating 20, so constructed as to present 1 megohm of resistance, does not exceed one hundred volts, or considerably less than one percent of the normal final anode voltage employed, in cathode-ray tube operation, so that anode 18 and final anode 21 are maintained at substantially equal potentials.

The invention affords great advantage in multiple electron gun devices such as color television picture tubes, particularly in view of the greater electrostatic charge build-up as compared with a single gun tube, since the quantity of such charge is determinative of the magnitude of discharge current.

Thus, an improved cathode-ray tube 10 utilizing a resistive coating 20 provides protection for the electron gun as well as for the associated external circuitry by preeluding spark-discharge and theresulting dielectric breakdown and high discharge currents. Moreover, this ob- 4 jective is accomplished by economical means which can be incorporated within the cathode-ray tube at an early stage in its manufacture and furthermore does not require any subsequent adjustment or replacement.

While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modification as fall within the true spirit and scope of the invention.

I claim:

1. A cathode-ray tube comprising within an evacuated envelope having a neck portion and a flared portion: a luminescent screen; means including a multi-electrode electron gun disposed in said neck portion for projecting an electron beam at said luminescent screen; a lowresistance final anode encompassing the path of said electron beam throughout substantially the entire length of said fiared portion of said envelope; and high resistance means interconnecting said final anode with a predetermined one of the electrodes in said electron gun for maintaining said anode and said electrode at substantially equal potentials while limiting spark discharge currents in said electron gun.

2. A cathode-ray tube in accordance with claim 1, in which said final anode is formed as a conductive coating on the inner wall of said envelope, and said high-resistance means includes a resistive coating on said wall in electrical contact with said conductive coating and further includes resilient contact means interconnecting said predetermined electrode and said resistive coating.

3. A cathode-ray tube in accordance with claim 2, in which the resistance of said resistive coating is at least of the order of one megohm.

References Cited in the file of this patent UNITED STATES PATENTS 2,151,992 Schwartz Mar. 22, 1939 2,275,864 Record Mar. 10, 1942 2,734,141 Hughes 2. Feb. 7, 1956 

